Unlike in other parts of our lexicon, the actual meaning of conjoined terms matters less than how they sound. This is especially true of those that are onomatopoeic, sounding like what they depict. Choo-choo. Click-clack. Clip-clop. Doo-wop. Boogie-woogie. Pitter-patter. In some cases, one of the words in conjoined terms exists primarily to rhyme or alliterate with the other. What is a dokey anyway, a nilly, a toity, a totsy, a poly, or a roly, for that matter? What’s the difference between a knick and a knack? A doo and a dad? A gee and a gaw? Higgledy and piggledy? Airy and fairy? Is it better to dilly or dally? To be a fuddy or a duddy? A riff or a raff? Who cares? We just like the cadence of such conjoined terms. Duper, after all, adds little to super except rhyme and rhythm. Rhyming is the main reason we talk of fat cats rather than fat dogs. In political palaver, the alliteration of flip-flop makes that term far more expressive than change positions.
Ralph Keyes, “Conjoined Terms”